The invention is in the field of electronic reproduction technology and is directed to a method for engraving printing forms for rotogravure on a print cylinder in an electronic engraving machine.
In an electronic engraving machine, an engraving element with an engraving stylus as a cutting tool moves along a rotating print cylinder in the axial direction. The engraving stylus controlled by an engraving control signal cuts a sequence of cups arranged in an engraving raster into the generated surface of the print cylinder. The engraving control signal is formed by superimposition of image signal values, which represent the gradations to be printed between xe2x80x9clightxe2x80x9d (white) and xe2x80x9cdarkxe2x80x9d (black), with a periodic raster signal. Whereas the raster signal effects a vibrating lifting motion of the engraving stylus for producing an engraving raster, the image signal values determine the geometric dimensions of the cups engraved into the generated surface of the print cylinder and, thus, the engraved gradations.
The engraving control signal must be calibrated so that thee engraved cups correspond to the gradations prescribed by the image signal values. For that purpose, test cups for predetermined gradations are engraved and interpreted for calibration in a test engraving, also called a test cut, that occurs before the actual engraving of the printing form.
So that the test cups are not visible on the printed product, the test cups in the test cut must be engraved in a cylinder region, referred to below as a test cut region, that does not overlap with the elements of the printing form to be engraved later.
The definition of a corresponding test cut region has hitherto been carried out by an operator directly at the engraving machine. Insofar as an adequate cylinder edge is available, a manual definition of the test cut region is unproblematical. When, however, an adequately large cylinder edge is lacking, then the operator must visually define a corresponding test cut region as exactly as possible between the elements of the printing form to be engraved, which is particularly difficult given printing forms for packaging printing since such a printing form comprises a multitude of relatively small elements.
The operator must work very carefully in order to avoid positioning errors in the manual definition of a test cut region, this leading to extended preparation times for the engraving. Over and above this, the known procedure has the disadvantage that it is not adequately suited for an automatic engraving of printing cylinders.
It is therefore an object of the present invention to improve a method for engraving printing forms for rotogravure on a print cylinder in an electronic engraving machine such that a faulty positioning of a test cut region suitable for the test cut ensues automatically on the print cylinder.
According to the method of the present invention for the line-by-line engraving of printing forms for rotogravure on a printing cylinder in an electronic engraving machine with an engraving element charged by an engraving control signal that engraves into the cylinder a sequence of cups arranged in an engraving raster, and wherein geometric dimensions of the cups represent gradations to be engraved, positions of elements to be engraved in the printing form are determined by a cylinder layout. Test cups are engraved into the print cylinder within at least one test cut region during a test cut occurring before the engraving of the printing form, geometric dimensions of the test cups are interpreted for calibration of the engraving control signal. Size and position of the test cut region are determined based on the cylinder layout for avoiding a faulty positioning of the test cut region on the print cylinder.
The invention is explained in greater detail below on the basis of FIGS. 1 through 3.